The present invention relates to a method of producing an etch-resistant polymer structure on a substrate using electron beam lithography.
More specifically, the present invention is concerned with the use of an electron beam to locally polymerize a layer of sterol deposited on a substrate to produce the etch-resistant polymer structure.
The fabrication of ultra-small scale semiconductor devices requires very high-resolution lithography techniques. The most frequent high-resolution lithography technique involved in such fabrication is the so-called polymeric resist-based lithography. PMMA (Poly(Methyl Methacrylate)) is a polymer that is currently used in the fabrication of such devices using electron beam lithography [xe2x80x9cMaterials and processes for nanometer lithographyxe2x80x9d, S. Mackie, S. P. Beaumont, Solid State Technology, vol. 28, August 1985, pp. 117-122]. This polymer is known mainly for its positive tone resist behavior, which allows the selective dissolution of the regions exposed to the electron beam while leaving intact the unexposed regions. Features with resolutions as small as 10 nm have been demonstrated with PMMA [xe2x80x9c10 nm linewidth electron beam lithography on GaAsxe2x80x9d H. G. Craighead, R. E. Howard, L. D. Jackel, P. M. Mankiewich, Applied Physics Letters, vol. 42, January 1983, pp. 38-40] and other comparable resists. However, fabrication of some semiconductor devices requires negative tone resists, in which the regions unexposed to the electron beam are dissolved while the exposed regions are left intact. Since the area to be exposed by the electron beam is smaller in the case of low-density patterns, negative tone resists reduce the exposure time, a major issue in large-scale device fabrication such as monolithic integrated circuits or in the case of clear field photomask fabrication using electron beam lithography.
Negative resists are common in microelectronics applications, as described in the following patents and articles:
xe2x80x9cHigh sensitivity negative electron resistxe2x80x9d, U.S. Pat. No. 3,770,433 (Bartlett et al.) issued on Nov. 6, 1973;
xe2x80x9cPlasma developable electron resist processxe2x80x9d, U.S. Pat. No. 4,386,152 granted to Juey H. Lai on May 31, 1983;
xe2x80x9cHigh resolution electron beam lithography using ZEP-520 and KRS resists at low voltagexe2x80x9d, D. M. Tanenbaum, C. W Lo, M. Isaacson, H. G. Craighead, M. J. Rooks, K. Y Lee, W. S. Huang, T. H. P. Chang, Journal of Vacuum Science and Technology B, vol. 14, November/December 1996, pp. 3829-3833; and
xe2x80x9cUltrahigh resolution of calixarene negative resist in electron beam lithographyxe2x80x9d, J. Fujita, Y. Ohnishi, Y. Ochiai, S. Matsui, Applied Physics Letters, vol. 68, February 1996, pp. 1297-1299.
In several negative resists, the electron beam is used to break bonds of the polymeric chains. This leaves free radicals to create cross-linking between the chains, and generates a non-soluble organic compound in the area exposed to the electron beam [xe2x80x9cEpoxy-polymer electron beam resistsxe2x80x9d, U.S. Pat. No. 3,916,035 (Brewer) issued on Oct. 28, 1975]. The resolution of both positive and negative tone resists is limited since the polymeric nature of the electron sensitive layer and the molecular dynamic behavior of the development process remove complete polymer chains. The polymer chains are entangled in a random manner, and the dimensions of the entanglement structure of the polymer as well as the diameter of the broken and disentangled polymeric chains are usually of the order of 5 nm ([xe2x80x9cElectron resistxe2x80x9d, U.S. Pat. No. 4,269,962 granted to J. Kalal, B. Bednar, J. Zachoval, J. Petr, Z. Pelcbauer and F. Svec on May 26, 1981] and [xe2x80x9cNanostructure technologyxe2x80x9d, T. H. P. Chang, D. P. Kem, E. Kratschmer, K. Y Lee, H. E. Luhn, M. A. McCord, S. A. Rishton, Y. Vladimirsky, IBM Journal of Research and Development, vol. 32, July 1988, pp. 462-492]). This impairs the achievement of structures with better resolution and smaller line edge roughness.
Another drawback of both the positive and negative tone resists currently available is the incompatibility of such resists with biological tissues. Of course, devices to be implanted in the human body such as cochlear implants, ocular implants and pain-suppressing implants are designed to come into direct contact with human organs. To obtain biocompatible devices, the electron-sensitive layers used for the patterning of the device elements must also be biocompatible. Otherwise, a thick encapsulating layer of biocompatible material has to be deposited on the devices to form a barrier for non-biocompatible material [xe2x80x9cMacroparticle distribution and chemical composition of laser deposited apatite coatingsxe2x80x9d, V. N. Bagratashvili, E. N. Antonov, E. N. Sobol, V. K Popov, S. M. Howdle, Applied Physic Letters, vol. 66, May 1985, pp. 2451-2453]. The presence of such an encapsulating layer obviously constitutes a limitation to the fabrication of some devices.
Finally, the most important drawback is the need for spin-coating such resists. Spin coating requires mostly flat surfaces with low relief. Abrupt and/or high structures on the surface of the substrate produces an uneven spun resist. This causes reduction in the achievable resolution. Recently, a method has been developed for fabricating an etch resistant metal/semiconductor compound using direct-write electron beam exposure ([xe2x80x9cFabrication of sub-micron suicide structures on silicon using resistless electron beam lithographyxe2x80x9d, U.S. Pat. No. 5,918,143 granted to J. Beauvais, D. Drouin and E. Lavallxc3xa9e on Jun. 29, 1999] and [xe2x80x9cMethod for fabricating submicron suicide structures on silicon using a resistless electron beam lithography processxe2x80x9d, D. Drouin, J. Beauvais, R. Lemire, and E. Lavallxc3xa9e, R. Gauvin, M. Caron, Applied Physics Letters, vol. 70, June 1997, pp. 3020-3023]. According to this method, the electron-sensitive layer can be evaporated on top of the substrate prior to electron beam exposure. Similar results have been attained using electron sensitive inorganic layers [xe2x80x9cFabrication of metallic structures in the 10 nm region using an inorganic electron beam resistxe2x80x9d, W Langheinrich, H. Beneking, Japanese Journal of Applied Physics, vol. 32, December 1993, pp. 6218-6223]. However, the sensitivity of such resists remains low, therefore restraining the use of this process to applications requiring low throughput such as mask fabrication.
In accordance with the present invention, there is provided a method of producing a structure of etch-resistant polymer on a substrate, comprising:
(a) depositing on a face of the substrate a layer of sterol capable of polymerizing to form the structure of etch-resistant polymer;
(b) exposing a first region of the layer of sterol to an electron beam to locally polymerize the layer of sterol and form the structure of etch-resistant polymer; and
(c) removing a second region of the layer of sterol which has not been exposed to the electron beam to leave on the face of the substrate only the structure of etch-resistant polymer.
The present invention also relates to a method of producing on a substrate a mask for lithography, comprising:
(a) depositing on a face of the substrate a radiation-absorbing layer;
(b) depositing on the radiation-absorbing layer a layer of sterol capable of polymerizing;
(c) exposing a first region of the layer of sterol to an electron beam to locally polymerize the layer of sterol and form a structure of etch-resistant polymer;
(d) removing a second region of the layer of sterol which has not been exposed to the electron beam to leave on the radiation-absorbing layer only the structure of etch-resistant polymer; and
(e) etching from the face of the substrate a region of the radiation-absorbing layer not covered by the structure of etch-resistant polymer.
The invention still further relates to a method of producing on a substrate a mask for X-ray lithography, comprising:
(a) depositing on a face of the substrate a first layer of etch-resistant material forming a first barrier to a particular etching process;
(b) depositing on the first layer a second layer of X-ray absorbing material;
(c) depositing on the second layer a third layer of etch-resistant material forming a second barrier to the particular etching process;
(d) depositing on the third layer a layer of sterol capable of polymerizing;
(e) exposing a first region of the layer of sterol to an electron beam to locally polymerize the layer of sterol and form a structure of etch-resistant polymer;
(f) removing a second region of the layer of sterol which has not been exposed to the electron beam to leave on the third layer only the structure of etch-resistant polymer;
(g) etching a region of the third layer not covered by the structure of etch-resistant polymer; and
(h) removing, by means of said particular etching process, a region of the second layer not covered by the structure of etch resistant polymer.
The foregoing and other objects, advantages and features of the present invention will become more apparent upon reading of the following non restrictive description of illustrative embodiments thereof, given by way of example only with reference to the accompanying drawings.